Integrated optics is the technique of manipulating light (photons) by miniature optical elements formed on a thin layer, like integrated circuits is the operating of electrons by numerous electronic elements formed on a thin layer.
The thin film optical waveguide is the basic structure of optical elements used in integrated optics. Among optical waveguides, the optical waveguide using a lithium niobate (ferroelectric) (LiNbO.sub.3) single crystal substrate has been extensively studied since the 1970s, because from LiNbO.sub.3 various optical elements can be fabricated e.g., the optical modulator/switch which uses the electro-optic effect.
The most extensively used and most developed technique for fabricating an optical waveguide, particularly having a LiNbO.sub.3 single crystal is the titanium (Ti) indiffusion method. This method is carried out at a high temperature (about 1000.degree. C.), with the result that the lithium ions are outdiffused to the surface of the crystal, thereby increasing its refractive index. In waveguides fabricated by this method, when an extraordinary wave is guided which uses a large electro-optic effect, there is a disadvantage that a surface guiding phenomenon occurs. Further, in such waveguides optical damages due to the photorefractive effect is produced from light in the visible light region, thereby making them difficult to be practical.
In the 1980s, came the development of the proton exchange method for fabricating an optical waveguide such as from LiNbO.sub.3 in which a chemical reaction is performed at a temperature lower than that used in the Ti indiffusion method (i.e. about 200.degree. C.)
In the proton exchange method a substrate is immersed in a benzoic acid (C.sub.6 H.sub.5 COOH) solution to exchange the lithium ions with protons. In this method, a large variation of the refractive index can be obtained, and the total time for the whole process can be shorter than the time in titanium indiffusion method. However, in the proton exchange method, the proton exchange reaction occurs abruptly; and, the resultant products have non-uniform characteristics. From the proton exchange fabrication process, optical waveguides have instabilities which become apparent and become a problem in the use thereof shortly after the fabrication. Therefore, this method is too fastidious to fabricate a single mode optical waveguide for practical use in integrated optics. The proton exchange method also suffers from a further problem in that the electro-optic effect which is characteristic of the lithium niobate is markedly lowered in waveguides from this method.
Recently, there have been proposals to overcome the above-described disadvantages of the proton exchange method. The first calls for a proton exchange method carried out such that benzoic acid solution is buffered with lithium ions, and is characterized in that the proton exchange reactions are moderated by means of the lithium ions. However, this proposal does not have practicality because the reaction time therein is too long to form the optical waveguide, and the reaction is sensitive to the lithium ion concentration in the solution.
A second proposal calls for a proton exchange method carried out such that, after the proton exchanges, the protons that have exchanged into the lithium niobate crystal are annealed therein by applying a temperature of over 350.degree. C. In this proposal, due to abrupt proton exchange reactions, the excessively exchanged protons gain heat energy which causes them to be outdiffused, with the result that the internal stress of the crystal structure is relaxed. However, in this proposal, the lithium ions, up to as many as the number of the exchanged protons during the exchange process, are transferred into the solution and cannot return to their original positions. In an attempt to overcome this phenomenon, only as many protons as are required for forming the optical waveguide are exchanged; and then, the protons have to be diffused only as deep as required, with the outdiffusion of the protons inhibited. However, this modification to the second proposal provides a process which is a very fastidious one.